Optical element, optical head, optical recording reproducing apparatus and optical recording/reproducing method

ABSTRACT

The invention presents an optical element with which an optical head can be configured, in which there is little deterioration of the correctional effect when the objective lens shifts, as well as an optical head and an optical recording/reproducing apparatus using such an optical element. The invention also presents a novel optical recording/reproducing apparatus and optical recording/reproducing method. The optical element, includes a first voltage application electrode  13 , a first opposing electrode  17  arranged in opposition to the first voltage application electrode  13 , and a first phase changing layer  15  arranged between the first voltage application electrode  13  and the first opposing electrode  17 . By changing a voltage between the first voltage application electrode  13  and the first opposing electrode  17 , a phase that converts plane waves into spherical waves is imparted on light that is incident on the first phase changing layer  15.

This application is a divisional of application Ser. No. 09/911,143,filed Jul. 23, 2001, which application is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical elements, optical heads andoptical recording/reproducing apparatuses using the same, and opticalrecording/reproducing methods.

2. Description of the Related Art

Digital versatile disks (DVDs) can record digital information at arecording density that is about six times as high as that of compactdisks (CDs), so that they are noted as high-capacity optical recordingmedia. In order to reproduce high-density DVDs, the wavelength of thelaser beam has to be shorter, and the-numerical aperture (NA) of theobjective lens has to be larger than for reproducing CDs. Therefore, alaser beam with a wavelength of 650 nm is used for reproducing DVDs(compared to 780 nm for CDs), and an objective lens with a NA of 0.6 isused (compared to 0.45 for CDs). Attempts have been made to increase therecording density by making the wavelength of the laser beam evenshorter and the NA even larger. However, when the wavelength isshortened and the NA of the objective lens is increased, the recordingand reproducing margins for positional deviations of the recording layerin the thickness direction become small. Consequently, in that case, itis necessary to perform a correction of spherical aberration.

Moreover, optical recording media provided with a plurality of recordinglayers have a high recording density, but in that case, the distancefrom the surface of the optical recording medium to the recording layer(also referred to as “base material thickness” in the following) variesfrom recording layer to recording layer, thus causing sphericalaberration. To correct this spherical aberration, an optical headcorrecting wavefront aberration (in particular, spherical aberration)with a liquid crystal element has been proposed (JP H10-269611A).

An example of this conventional optical head is explained with referenceto FIG. 13. FIG. 13 schematically illustrates the configuration of aconventional optical head 200 (also referred to as an “optical pickup”).As shown in FIG. 13, the optical head 200 includes a light source 201, apolarizing beam splitter 202, a liquid crystal panel 203, a λ/4 plate204, an objective lens 205, a focusing lens 206, an optical detector207, a base material thickness sensor 208, and an optical elementdriving circuit 209. Signals are recorded on or reproduced from theoptical disk 210 with this optical head 200.

The light source 201 is made of a semiconductor laser element, whichemits coherent light for recording/reproducing towards the recordinglayer of the optical disk 210. The polarizing beam splitter 202 servesas an element for separating the light. The liquid crystal panel 203includes a plurality of electrodes 203 a to 203 d, arrangedconcentrically as shown in FIG. 14, which change the refractive index ofthe liquid crystal by applying different voltages to the electrodes,thus correcting aberration. The λ/4 plate 204 is made of a birefringentmaterial, and converts linearly polarized light into circularlypolarized light. The objective lens 205 focuses light on the recordinglayer of the optical disk 210. The focusing lens 206 focuses light thathas been reflected by the recording layer of the optical disk 210 on theoptical detector 207. The optical detector 207 receives the light thathas been reflected by the recording layer of the optical disk 210 andconverts it into an electrical signal.

The following is an explanation of the operation of this optical head.Linearly polarized light that is emitted from the light source 201passes through the polarizing beam splitter 202 and enters the liquidcrystal panel 203. If the recording layer of the optical disk 210 isarranged at a position that is different from the design value, the basematerial thickness sensor 208 detects this deviation, and outputs thisdeviation to the optical element driving circuit 209. Based on thereceived deviation, the optical element driving circuit 209 drives theliquid crystal panel 203 to correct the wavefront aberration caused bythis deviation. Consequently, a wavefront aberration that corrects thewavefront aberration caused by the deviation of the base materialthickness (third-order spherical aberration) is imparted on the lightentering the liquid crystal panel 203.

The following is a more detailed explanation of a method for correctingspherical aberration with the liquid crystal panel 203. First, FIG. 15shows the phase distribution when the base material thickness of theoptical disk 210 deviates from the design value (i.e. the optimum basematerial thickness). FIG. 15 shows the phase distribution for a laserbeam wavelength of 405 nm, an NA of the objective lens of 0.85, anoptimum base material thickness of the optical disk 210 of 0.1 mm, and abase material thickness deviation of 0.01 mm, illustrating thedistribution of the wavefront aberration on the recording layer of theoptical disk 210 at the best image point. If a phase correcting thisdistribution completely is added to the laser beam, then the spot of thelaser beam on the optical disk 210 can be constricted to the diffractionlimit, even though the base material thickness of the optical disk 210deviates from the optimum base material thickness.

In order to correct the wavefront aberration in FIG. 15, a phase changecanceling the wavefront aberration in FIG. 15 should be imparted on thelaser beam. That is to say, the optical path length should be partiallychanged. With a liquid crystal, the optical path length can be partiallychanged by changing the voltage applied to the liquid crystal, becauseits refractive index depends on the voltage applied to it. Consequently,the spherical aberration shown in FIG. 15 can be corrected by applying asuitable voltage to the electrodes 203 a to 203 d shown in FIG. 14.

However, in the optical head 200, spherical aberration is corrected bygenerating third-order spherical aberration, so that the effect ofcorrecting spherical aberration is poor when the center of the objectivelens 205 deviates from the center of the electrodes 203 a to 203 d ofthe liquid crystal panel 203. That is to say, if the objective lens 205and the liquid crystal panel 203 are arranged in separation from eachother, then the center of the objective lens 205 deviates from thecenter of the electrodes 203 a to 203 d of the liquid crystal panel 203,due to the shifting of the objective lens 205 in accordance with theeccentricity of the optical disk 210, thus worsening the correctiveeffect.

FIG. 16 illustrates the relation between the deviation of the center ofthe objective lens 205 from the center of the electrodes 203 a to 203 dand the aberration after the correction, when the wavelength of thelaser beam is 400 nm, the NA is 0.85, and the base material thickness ofthe optical disk 210 deviates 10 μm from the design value (0.1 mm).

As shown in FIG. 16, when the center of the objective lens 205 deviatesfrom the center of the electrodes 203 a to 203 d, the corrective effectdeteriorates. This is because, due to the deviation between the centers,the spherical aberration generated by the liquid crystal panel 203causes coma aberration. In order to prevent deviation of the centers, itis necessary to form the liquid crystal panel 203 in one piece with theobjective lens 205.

However, when the liquid crystal panel 203 is formed in one piece withthe objective lens 205, it is difficult to make the optical headthinner. Also, since it becomes necessary to shift the liquid crystalpanel 203 together with the objective lens 205, the frequency response(sensitivity) of the actuator drops. Moreover, the wiring to drive theliquid crystal panel 203 makes manufacture of the actuator more complex,so that it becomes difficult to lower the costs.

As a method for correcting spherical aberration, it has been proposed tocorrect spherical aberrations by arranging two lenses on the opticalaxis and changing the spacing of the lenses (see JP 2000-131603A). Inthis method, changing the spacing of the lenses imparts a phase changeon the light passing through the lenses, which changes parallel lightinto divergent or convergent light, thus correcting sphericalaberration. However, this method necessitates a mechanical means forchanging the spacing of the lenses in accordance with the deviation ofthe base material thickness, which makes miniaturization of the opticalhead difficult. Moreover, to prevent the occurrence of coma aberration,the centers of the two lenses have to be matched precisely, which makesit difficult to manufacture the optical head at low cost. Furthermore,in this method, the spacing of the lenses is changed on the opticalaxis, so that the optical system becomes magnifying or contracting. As aresult, there is the problem that the transmission efficiency of lightthat is incident on the lenses for correcting spherical aberrationchanges, and the rim intensity of the light changes.

The present invention has been developed in view of these problems, andit is a first object of the present invention to provide an opticalelement with which an optical head can be configured, in which there islittle deterioration of the correctional effect when the objective lensshifts, as well as an optical head and an optical recording/reproducingapparatus using such an optical element. It is a second object of thepresent invention to present a novel optical recording/reproducingapparatus and optical recording/reproducing method.

SUMMARY OF THE INVENTION

In order to achieve these objects, an optical element in accordance withthe present invention includes a first voltage application electrode; afirst opposing electrode arranged in opposition to the first voltageapplication electrode; and a first phase changing layer arranged betweenthe first voltage application electrode and the first opposingelectrode; wherein, by changing a voltage between the first voltageapplication electrode and the first opposing electrode, a phase thatconverts plane waves into spherical waves is imparted on light that isincident on the first phase changing layer. With this optical element,it is possible to configure an optical head, in which there is littledeterioration of the corrective effect when the objective lens isshifted.

In the optical element, it is preferable that at least one electrodeselected from the first voltage application electrode and the firstopposing electrode is arranged on a curved surface. With thisconfiguration, it is not necessary to partition the voltage applicationelectrode, so that wiring becomes easy.

In the optical element, it is preferable that the first phase changinglayer is made of a material whose refractive index changes when applyinga voltage. With this configuration, it is easy to change the phase ofincident light.

In the optical element, it is preferable that the first phase changinglayer is made of a liquid crystal. With this configuration, the voltagenecessary to change the phase of the incident light can be lowered.

In the optical element, it is preferable that the first phase changinglayer is made of a material whose volume changes when subjected to avoltage. With this configuration, it is easy to change the phase ofincident light. Furthermore, the phase can be changed independently fromthe polarization direction.

In the optical element, it is preferable that the first phase changinglayer is made of PLZT. With this configuration, the element can be madethin.

In the optical element, it is preferable that the first voltageapplication electrode includes a plurality of segment electrodes. Withthis configuration, manufacture becomes easy.

It is preferable that the optical element further includes a secondvoltage application electrode; a second opposing electrode arranged inopposition to the second voltage application electrode; and a secondphase changing layer arranged between the second voltage applicationelectrode and the second opposing electrode; wherein, by changing avoltage between the second voltage application electrode and the secondopposing electrode, a phase that converts plane waves into sphericalwaves is imparted on polarized light that is perpendicular to thepolarization direction of the light that is incident on the first phasechanging layer. With this configuration, an optical head can beconfigured in which the corrective effect does not deteriorate whenshifting the objective lens in a optical polarization system.

In this optical element, it is preferable that at least one electrodeselected from the second voltage application electrode and the secondopposing electrode is arranged on a curved surface.

In this optical element, it is preferable that the second phase changinglayer is made of a material whose refractive index changes when applyinga voltage between the second voltage application electrode and thesecond opposing electrode.

In this optical element, it is preferable that the second phase changinglayer is made of a material whose volume changes when applying a voltagebetween the second voltage application electrode and the second opposingelectrode.

In this optical element, it is preferable that the second voltageapplication electrode includes a plurality of segment electrodes.

In accordance with the present invention, an optical head for recordingor reproducing signals on an optical recording medium includes a lightsource; an optical element arranged between the optical recording mediumand the light source; and an objective lens arranged between the opticalrecording medium and the optical element; wherein the, optical elementis the above-described optical element of the present invention. Withthis optical head, spherical aberration is corrected by the combinationof an optical element of the present invention and an objective lens, sothat there is little deterioration in the corrective effect when theobjective lens shifts. Furthermore, the manufacture of this optical headis easy.

It is preferable that this optical head further includes an N/4wavelength plate (wherein N is an odd number of one or greater) arrangedbetween the optical element and the objective lens. With thisconfiguration, the utilization efficiency of the light emitted by thelight source is high, so that it is easy to record or reproduce signalon the optical recording medium.

In accordance with the present invention, a first opticalrecording/reproducing apparatus for recording or reproducing signals onan optical recording medium includes an optical head for recording orreproducing signals on an optical recording medium; the optical headincluding a light source; an optical element arranged between theoptical recording medium and the light source; and an objective lensarranged between the optical recording medium and the optical element;wherein the optical element is the above-described optical element ofthe present invention. This optical recording/reproducing apparatus usesan optical element of the present invention, so that signals can berecorded or reproduced with high reliability. Furthermore, themanufacture of this optical recording/reproducing apparatus is easy.

In accordance with the present invention, a second opticalrecording/reproducing apparatus for recording or reproducing signals ona first optical recording medium including only one recording layer andon a second optical recording medium including a plurality of recordinglayers, includes an optical head for recording or reproducing signals onthe first and second optical recording media, the optical head includinga light source; and a spherical aberration correction means arrangedbetween the optical recording medium and the light source; wherein adistance from a surface of the first optical recording medium to the onerecording layer A included in the first optical recording medium issubstantially the same as the distance from a surface of the secondoptical recording medium to one recording layer B included in the secondoptical recording medium. With this second optical recording/reproducingapparatus, the time until recording or reproducing can be shortened,because it is not necessary to classify the optical recording medium.

In this second optical recording/reproducing apparatus, it is preferablethat, in an initial state before recording or reproducing signals on thefirst or the second optical recording medium, the spherical aberrationcorrection means is driven so as to correct spherical aberration of therecording layer A. With this configuration, focus control and recordingor reproducing can be carried out properly for the second opticalrecording medium.

In the second optical recording/reproducing apparatus, it is preferablethat when recording or reproducing signals on a recording layer C of thesecond optical recording medium that is different from the recordinglayer B, the spherical aberration correction means is driven so as tocorrect spherical aberration of that recording layer C.

It is preferable that the second optical recording/reproducing apparatusfurther includes a focus control means; and that in the initial state,after driving the spherical aberration correction means so as to correctspherical aberration of the recording layer A, focus control isperformed with the focus control means.

In the second optical recording/reproducing apparatus, it is preferablethat administrative information of the second optical recording mediumis stored in the recording layer B.

In accordance with the present invention, a third opticalrecording/reproducing apparatus for recording or reproducing signals ona first optical recording medium including only one recording layer andon a second optical recording medium including a plurality of recordinglayers, includes a light source; a spherical aberration correction meansarranged between the optical recording medium and the light source; afocus error detection means; and a focus control means; wherein, in aninitial state before recording or reproducing signals on the first orthe second optical recording medium, the spherical aberration correctionmeans is driven so as to correct spherical aberration of the recordinglayer included in the first optical recording medium, then, a focuserror is detected with the focus error detection means, and focuscontrol is performed with the focus control means, based on the detectedfocus error. With this third optical recording/reproducing apparatus,the time until recording or reproducing can be shortened, because it isnot necessary to classify the optical recording medium.

In accordance with the present invention, a fourth opticalrecording/reproducing apparatus for recording or reproducing signals ona first optical recording medium including only one recording layer andon a second optical recording medium including a plurality of recordinglayers, includes a light source; a spherical aberration correction meansarranged between the optical recording medium and the light source; afocus error detection means; and a focus control means; wherein, if itis known whether the optical recording medium subjected to recording orreproducing is a first optical recording medium or a second opticalrecording medium, then the spherical aberration correction means isdriven so as to correct spherical aberration at a standard base materialthickness of the recording layer subjected to recording or reproducing,then, a focus error is detected with the focus error detection means,and focus control is performed with the focus control means, based onthe detected focus error. With this fourth optical recording/reproducingapparatus, smooth focus control becomes possible. It should be notedthat “standard base material thickness” means the design value of thebase material thickness.

In accordance with the present invention, in a firstrecording/reproducing method for recording or reproducing signals withan optical recording/reproducing apparatus on a first optical recordingmedium including only one recording layer and on a second opticalrecording medium including a plurality of recording layers, the opticalrecording/reproducing apparatus includes a spherical aberrationcorrection means; a distance from a surface of the first opticalrecording medium to the one recording layer A included in the firstoptical recording medium is substantially the same as the distance froma surface of the second optical recording medium to one recording layerB included in the second optical recording medium; and the methodincludes a first step of driving the spherical aberration correctionmeans so as to correct spherical aberration of the recording layer A.

It is preferable that this first recording/reproducing method furtherincludes a second step, carried out after the first step, wherein, whenrecording or reproducing signals on a recording layer C of the secondoptical recording medium that is different from the recording layer B,the spherical aberration correction means is driven so as to correctspherical aberration of that recording layer C.

In accordance with the present invention, in a second opticalrecording/reproducing method for recording or reproducing signals withan optical recording/reproducing apparatus on a first optical recordingmedium including only one recording layer and on a second opticalrecording medium including a plurality of recording layers, the opticalrecording/reproducing apparatus includes a spherical aberrationcorrection means, a focus error detection means, and a focus controlmeans; and the method includes a first step of driving the sphericalaberration correction means so as to correct spherical aberration of therecording layer included in the first optical recording medium; a secondstep of detecting a focus error with the focus error detection means;and a third step of performing focus control with the focus controlmeans, based on the detected focus error; wherein the first, second andthird step are carried out before recording or reproducing. With thissecond optical recording/reproducing method, the time until recording orreproducing can be shortened, because it is not necessary to classifythe optical recording medium.

In accordance with the present invention, in a third opticalrecording/reproducing method for recording or reproducing signals withan optical recording/reproducing apparatus on a first optical recordingmedium including only one recording layer and on a second opticalrecording medium including a plurality of recording layers, the opticalrecording/reproducing apparatus includes a spherical aberrationcorrection means, a focus error detection means, and a focus controlmeans; and the method includes a first step of acquiring informationabout whether the optical recording medium subjected to recording orreproducing is a first optical recording medium or a second opticalrecording medium; a second step of driving the spherical aberrationcorrection means so as to correct spherical aberration at a standardbase material thickness of the recording layer subjected to recording orreproducing, based on that information; a third step of detecting afocus error with the focus error detection means; and a fourth step ofperforming focus control with the focus control means, based on thedetected focus error; wherein the first, second, third, and fourth stepare carried out before recording or reproducing. With this third opticalrecording/reproducing method, smooth focus control becomes possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing an example of an opticalelement of the present invention.

FIG. 2 is a top view showing an example of a voltage applicationelectrode in an optical element of the present invention.

FIG. 3 is a cross-sectional view showing another example of an opticalelement of the present invention.

FIG. 4 is a graph showing an example of the relationship between thedistance from the optical axis and the phase necessary to correctspherical aberration.

FIG. 5 schematically illustrates the configuration of an example of anoptical head of the present invention.

FIG. 6 is a graph showing an example of the relationship between thebase material thickness and the aberration.

FIG. 7 is a graph showing an example of the relationship between thelens shift and the aberration.

FIG. 8 is a graph showing another example of the relationship betweenthe lens shift and the aberration.

FIG. 9 schematically illustrates another example of an optical head ofthe present invention.

FIG. 10 is a cross-sectional view showing another example of an opticalelement of the present invention.

FIG. 11 schematically illustrates the configuration of an example of anoptical recording/reproducing apparatus of the present invention.

FIG. 12A illustrates a first example and FIG. 12B illustrates a secondexample of the base material thickness in a first optical recordingmedium and a second optical recording medium, which are recorded orreproduced with the optical recording/reproducing apparatus of thepresent invention.

FIG. 13 schematically illustrates the configuration of an example of aconventional optical head.

FIG. 14 is a plan view showing an example of the electrodes on a liquidcrystal panel used in a conventional optical element.

FIG. 15 shows an example of the distribution of wavefront aberrationwhen the actual base material thickness deviates from the design value.

FIG. 16 is a graph showing an example of the relationship between lensshifts and aberrations when the actual base material thickness deviatesfrom the design value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are explained below with referenceto the drawings. Note that similar parts are marked by identical numbersand redundant explanations have been omitted.

Embodiment 1

In Embodiment 1, an example of an optical element of the presentinvention is described. FIG. 1 shows a schematic cross-section of anoptical element 10 of Embodiment 1.

Referring to FIG. 1, the optical element 10 is provided with a firstsubstrate 11, a second substrate 12 disposed substantially parallel tothe first substrate 11, a voltage application electrode 13 (firstvoltage application electrode) disposed between the first substrate 11and the second substrate 12, a translucent resin film 14, a liquidcrystal 15 (first phase changing layer), a translucent resin film 16, anopposing electrode 17 (first opposing electrode), and a sealing resin18. The liquid crystal 15 is sealed by the sealing resin 18.

The first substrate 11 and the second substrate 12 are translucentsubstrates made from glass, for example.

The voltage application electrode 13 applies a predetermined voltage tothe liquid crystal 15. The voltage application electrode 13 is formed onthe primary surface of the inner side (liquid crystal 15 side) of thefirst substrate 11. The voltage application electrode 13 is made from atranslucent, conductive material, such as indium tin oxide (ITO). FIG. 2shows a top view of the voltage application electrode 13. The voltageapplication electrode 13 includes a plurality of segment electrodes 13 ato 13 g disposed in concentric circles. Note that in FIG. 2 the voltageapplication electrode 13 is shown as made up of seven segment electrodes13 a to 13 g, but there is no particular limitation with regard to thenumber of the segment electrodes.

The opposing electrode 17 is formed on the second substrate 12 such thatit opposes the voltage application electrode 13. Together with thevoltage application electrode 13, the opposing electrode applies apredetermined voltage to the liquid crystal 15. The opposing electrode17 is made from a translucent, conductive material, such as ITO. Theopposing electrode 17 is formed at least on the entire surface of thoseparts of the primary surface on the inner side (liquid crystal 15 side)of the second substrate 12 that oppose the segment electrodes 13 a to 13g.

The translucent resin films 14 and 16 are alignment films for aligningthe liquid crystal 15 in a predetermined direction, and are made of, forexample, polyvinyl alcohol films. By rubbing the translucent resin film14 or 16, the liquid crystal 15 can be aligned in a predetermineddirection.

The liquid crystal 15 functions as a phase changing layer that changesthe phase of incident light. The liquid crystal 15 is made of, forexample, a nematic liquid crystal. By changing the voltage between thevoltage application electrode 13 and the opposing electrode 17, therefractive index of the liquid crystal 15 can be changed, and this makesit possible to change the phase of the incident light.

The sealing resin 18 is for sealing the liquid crystal 15, and is madefrom an epoxy resin, for example.

In the optical element 10, by changing the voltage between each of thesegment electrodes 13 a to 13 g and the opposing electrode 17, therefractive index of the liquid crystal 15, which is a phase changinglayer, is partially changed, and a phase that converts plane waves intospherical waves is imparted on the light entering the liquid crystal 15.More specifically, as an example, a voltage of 0V can be applied to theopposing electrode, 0V to the segment electrode 13 a, 0.5V to thesegment electrode 13 b, 1V to the segment electrode 13 c, 1.5V to thesegment electrode 13 d, 2V to the segment electrode 13 e, 2.5V to thesegment electrode 13 f, and 3V to the segment electrode 13 g. In thisway, by increasing the voltage applied to the segment electrodes fromthe center of the voltage application electrode 13 outward, the phasedifference increases when progressing outward from the center of theoptical element 10, and it becomes possible to convert plane waves intospherical waves. With the optical element 10, as is explained inEmbodiment 2, an optical head can be configured in which the correctiveeffect is not lowered when shifting the objective lens.

At least one electrode chosen from the voltage application electrode 13and the opposing electrode 17 can be disposed on a curved surface (thesame applies for the embodiments below as well). FIG. 3 shows aschematic cross-section of an optical element 10 a, in which theopposing electrode 17 is disposed on the curved surface. In the opticalelement 10 a, the inner surface of the second substrate 12 is concave,and the opposing electrode 17 is formed on the surface thereof In theoptical element 10 a, the voltage application electrode 13 is notdivided up into a plurality of segment electrodes, but is formed as asingle surface. As explained in Embodiment 2 this enables theconfiguration of an optical head in which corrective effects do notparticularly decline even if the objective lens moves.

JP2001-143303A discloses another example of an optical element impartinga smooth phase change. This optical element includes a voltageapplication electrode made of a transparent electrode with a large sheetresistance and a voltage supply potion with a resistance that is lowerthan that of the transparent electrode. When a voltage is applied fromthe outside to the voltage supply potion connected to a potion of thetransparent electrode, a voltage drop occurs, because the sheetresistance of the transparent electrode is large, and the voltagedecreases smoothly with increasing distance from the voltage supplypotion. Thus, the voltage between the voltage application electrode andthe opposing electrode changes smoothly, and accordingly the refractiveindex of the phase changing layer changes smoothly as well.Additionally, methods other than rubbing may be used to control thealignment of the liquid crystal 15 (the same applies for the otherembodiments below as well). For example, the films themselves may begiven an alignment by using oblique vapor deposition. Additionally, itis also possible to control the alignment of the liquid crystal byforming grooves in the substrate.

Furthermore, it is also possible to use another material instead of theliquid crystal 15 to form the phase changing layer (the same applies forembodiments below as well). For the phase changing layer, it is possibleto use material whose refractive index or a material whose volumechanges with the voltage between the voltage application electrode 13and the opposing electrode 17. As a material whose volume changes withvoltage, it is possible to use PLZT (a transparent crystal of aperovskite structure that includes lead oxide, lanthanum, zirconiumoxide, and titanium oxide). PLZT is a solid, so unlike liquid crystalsis does not require a substrate or sealing resin film. For this reason,using PLZT allows making the optical element thinner.

Embodiment 2

Embodiment 2 describes an optical head of the present invention, whichuses the optical element described in Embodiment 1. FIG. 5 schematicallyshows the configuration of an optical head 50 of Embodiment 2.

Referring to FIG. 5, the optical head 50 is provided with a light source51, a diffraction grating 52, a collimating lens 53, an optical element54, an objective lens 55, a base material thickness sensor 56, a firstphotodetector 58, and a second photodetector 59. The optical element 54is driven by an optical element driving circuit 57. The collimating lens53 and the objective lens 55 form an optical focusing system. Theoptical head 50 records or reproduces a signal on an optical recordingmedium 60. The optical element 54 is an optical element as described inEmbodiment 1.

For the light source 51, it is possible to use a semiconductor laserelement. The light source 51 outputs a laser beam 61 (coherent light)for recording and reproducing toward the recording layer of the opticalrecording medium 60. The diffraction grating 52 has a zero-orderdiffraction efficiency of approximately 50%, and a ±first-orderdiffraction efficiency of approximately 50%. The diffraction grating 52can be formed by using photolithography to form a predetermined resistpattern on a glass surface, after which the glass is etched. The opticalelement 54 imparts on the light entering the optical element 54 a phasethat converts plane waves into spherical waves. For the firstphotodetector 58 and the second photodetector 59, it is possible to usephotodiodes.

The objective lens 55 focuses the laser beam 61 on the recording layerof the optical recording medium 60. The first photodetector 58 receivesthe +1 order light that has been diffracted by the diffraction grating52 from the laser beam 61 reflected by the recording layer of theoptical recording medium 60, and converts this light into an electricsignal. The second photodetector 59 receives the −1 order lightdiffracted by the diffraction grating 52 from the laser beam 61reflected by the recording layer of the optical recording medium 60, andconverts this light into an electric signal.

The base material thickness sensor 56 detects deviations between thepreviously set base material thickness and the actual base materialthickness, and outputs a signal depending on that deviation. For thebase material thickness sensor 56, the sensors described in JPH10-334575A and U.S. Pat. No. 6,115,336 can be used, for example. Thebase material thickness sensor 56 specifically includes a light source,a first optical system that irradiates the optical recording medium(measurement object) with the light emitted from the light source, and asecond optical system that guides light reflected from the opticalrecording medium into a light receiving element. This light source ismade from a laser, LED, or a lamp. The first and second optical systemsare made of convex lenses or a combination of convex lenses and concavelenses. With this configuration, the signal outputted from the lightreceiving element depends on the base material thickness. Additionally,JP 2000-171346A describes another separate method for detecting the basematerial thickness. In this method, spherical aberrations are detectedbased on the focusing position of a first light beam on the side nearthe optical axis of the light reflected from the optical recordingmedium, and the focusing position of a second light beam further outwardthan the first light beam.

The optical element driving circuit 57 drives the optical element 54such that it corrects deviations in the base material thickness based onthe base material thickness deviation inputted by the base materialthickness sensor 56.

The operation of the optical head 50 shall be explained with referenceto FIG. 5. A portion of the linear polarized light emitted from thelight source 51 passes through the diffraction grating 52 and enters thecollimating lens 53, which turns it into parallel light. This parallellight enters the optical element 54. When there is a deviation betweenthe design value and the base material thickness of the opticalrecording medium 60, the base material thickness sensor 56 outputs asignal in accordance with the amount of that deviation, and the signalis inputted to the optical element driving circuit 57. Based on theinputted signal, the optical element driving circuit 57 outputs to theoptical element 54 a signal that is necessary to correct the wavefrontaberration that occurs when there is a deviation in the base materialthickness of the optical recording medium 60. By doing this, a wavefrontaberration, which imparts a phase (power component) that converts theparallel light into divergent light, or a phase that converts theparallel light into convergent light, is imparted on the light thatenters the optical element 54, depending on the sign of the deviation ofthe base material thickness.

Light that has passed through the optical element 54 enters theobjective lens 55 as non-parallel light, thereby causing sphericalaberration. This spherical aberration corrects the spherical aberrationcaused by deviations between the base material thickness of the opticalrecording medium 60 and the design value. That is to say, the opticalelement 54 imparts a phase on the incident light such that a desiredspherical aberration occurs when it enters the objective lens 55. Thisforms a light spot on the optical recording medium 60 withoutaberration, that is, focused to the diffraction limit.

Next, the light reflected from the optical recording medium 60 turnsinto light that contains the wavefront aberration caused when there is adeviation between the base material thickness of the optical recordingmedium 60 and the design value, but the wavefront aberration iscorrected by the objective lens 55 and the optical element 54. The lightthat has passed through the optical element 54 then passes through thecollimating lens 53, and is diffracted by the diffraction grating 52.After this, diffracted +1 order light is irradiated onto the firstphotodetector 58, and diffracted −1 order light is irradiated onto thesecond photodetector 59. The first photodetector 58 outputs a focuserror signal, which indicates the focusing condition of the light on theoptical recording medium 60, and a tracking error signal, whichindicates the irradiation position of the light. The secondphotodetector 59 outputs a signal regarding information recorded on theoptical recording medium 60.

The focus error signal outputted from the first photodetector 58 isinputted to a focus control circuit, which is not shown in the drawings.The focus control circuit controls the position of the objective lens 55in the direction of the optical axis, based on the focus error signal,such that light is focused on the optical recording medium 60 in a stateof focus. Furthermore, the tracking error signal is inputted to atracking control circuit, which is not shown in the drawings. Thetracking control circuit controls the position of the objective lens 55,based on the tracking error signal, such that it focuses the light onthe desired track of the optical recording medium 60. The position ofthe objective lens 55 is controlled with actuators 62.

Next, the operation of the optical element 54 is explained with anexample of how the optical element 10 is used. In the optical element10, the refractive index of the liquid crystal 15, which is a phasechanging layer, is changed by changing the voltage between each of thesegment electrodes 13 a to 13 g and the opposing electrode 17, and aphase that converts plane waves into spherical waves is imparted on thelight entering the liquid crystal 15. In order to convert the planewaves into spherical waves, a phase P that satisfies the followingequation (1) should be imparted on the incident light.(a−P)² +r ² =a ²   (1)

Here, a is a constant, and r is the distance from the center of theoptical axis. FIG. 4 shows one example of a phase distribution thatsatisfies equation (1) when the wavelength of the laser beam is 405 nm,the NA is 0.85, the design value of the base material thickness (thatis, the optimum base material thickness) is 0.1 mm, and the deviation ofthe base material thickness from the design value is 10 μm.

FIG. 6 shows the relationship between the base material thickness andthe aberration after correction, when a segment electrode with fortyconcentric circles generates the phase shown in FIG. 4. For comparison,FIG. 6 also shows the aberration when no correction is carried out. FIG.7 shows lens shift properties when a deviation in base materialthickness of 10 μm is corrected. The horizontal axis in FIG. 7 marks thedeviation (lens shift) between the center of the optical axis and thecenter of the objective lens (which corresponds to the deviation betweenthe center of the segment electrode with forty concentric circles andthe center of the objective lens).

In the optical head 50, a phase (power component) converting plane wavesinto spherical waves is imparted on the light that has entered theoptical element 54, and together with the objective lens causesspherical aberration, thereby achieving correction. Consequently, thisis different from directly imparting on the light that has entered theoptical element the third-order spherical aberrations that should becorrected, and as shown in FIG. 7, the optical head 50 becomes extremelyresistant against lens shifts.

As disclosed in JP H10-360545A, for the optical element 54, it ispossible to use an arrangement of forming thin film resistances on theoptical element to voltage, divide the signal applied from the outside,and applying these divided voltages to the respective segmentelectrodes. With this arrangement, even with a high number of segmentelectrodes like forty, the voltage applied from outside can be splitthree ways, including the voltage applied to the opposing electrode.

When using the optical element 10, the necessary phases can be generatedin approximation to the plurality of segment electrodes. On the otherhand, by using an optical element 10 a it is possible to generatenecessary phases according to their actual shape. In the optical element10 a, the liquid crystal 15, which is a phase changing layer, is formedto be convex, so that it is possible to impart a continuously changingphase on the incident light without partitioning the voltage applicationelectrode.

Consequently, by using the optical element 10 a, it is possible toreproduce the power components with fidelity. In this case, it ispossible to reduce the aberration after correction to substantiallyzero, because no high-order aberrations are caused. FIG. 6 shows therelationship between the base material thickness and the aberrationafter correction, when the optical element 10 a is used. FIG. 8 showsthe lens shift properties when a deviation in the base materialthickness of 10 μm is corrected with the optical element 10 a. Also inthis case, the lens shift properties are extremely favorable, as withthe approximate correction by the segment electrodes.

As explained above, the optical head 50 of Embodiment 2 has favorablelens shift properties when correcting spherical aberrations. Moreover,the objective lens and the optical element can be disposed without beingformed in one piece, so that with the optical head 50, it is possible toread with high reliability signals recorded on an optical recordingmedium having a deviation in the base material thickness. Also, theoptical head 50 can be made even thinner. Also, because the opticalelement need not be installed in the actuator, it is possible to preventa drop in the frequency response (sensitivity) of the actuator, and tomanufacture at low costs and with simplified wiring. Furthermore, theoptical head 50 generates the power components electrically, in contrastto a conventional optical head that includes two lenses and mechanicallychanges the spacing between the lenses to generate the power componentswith the optical head 50. Therefore, the optical head 50 is suitable forminiaturization. Furthermore, with the optical head 50 it is possible toprevent changes in the rim intensity of the light, because thetransmission efficiency of light that has entered the optical elementdoes not change.

Embodiment 3

Embodiment 3 describes another example of the optical element andoptical head of the present invention. The optical head of Embodiment 3differs from the optical head of Embodiment 2 in that it uses an opticalpolarization system, which includes a polarized holographic opticalelement and a λ/4 plate, and in that it uses an optical element adaptedto the optical polarization system. Unless explained otherwise, partsgiven the same numerals as parts explained in Embodiment 1 andEmbodiment 2 have similar functions to the parts described in Embodiment1 and Embodiment 2.

FIG. 9 schematically shows the configuration of an optical head 90 ofEmbodiment 3. Referring to FIG. 9, the optical head 90 includes a lightsource 51, a collimating lens 53, an objective lens 55, a base materialthickness sensor 56, a first photodetector 58, a second photodetector59, a polarized holographic optical element 91, an optical element 100,and a λ/4 plate 93. The optical element 100 is driven by the opticalelement driving circuit 57.

The polarized holographic optical element 91 has the function ofseparating the polarized light. The polarized holographic opticalelement 91 acts on ordinary light rays as a diffraction grating, butdoes not act on extraordinary light rays (approximately 100%transmissivity). For the polarized holographic optical element 91, it ispossible to use the hologram disclosed in JP H6-27322A. This polarizedholographic optical element can be formed by proton exchanging apredetermined portion of a birefringent lithium nitrate substrate, andthen etching the proton exchanged portions.

The optical element 100 corrects aberrations by changing the refractiveindex of the liquid crystal. Further details regarding the opticalelement 100 will be explained later.

The λ/4 plate 93 is made of a quartz, for example. The λ/4 plate 93 is anon-linear optical element that converts linearly polarized light,emitted from the light source 51, into circularly polarized light, andconverts light reflected by the recording layer of the optical recordingmedium 60 into linearly polarized light with a polarization directionthat is different from that of when it was irradiated. It is alsopossible to use an N/4 plate (N being an odd number greater than one)instead of the λ/4 plate.

Next, the operation of the optical head 90 is explained with referenceto FIG. 9. Linearly polarized light (the laser beam 61) emitted from thelight source 51 passes through the polarized holographic optical element91 with a transmission-efficiency of approximately 100% and enters thecollimating lens 53, which turns it into parallel light. This parallellight enters the optical element 100.

At this point, when there is a deviation between the design value andthe actual base material thickness of the optical recording medium 60,the base material thickness sensor 56 outputs a signal corresponding tothat deviation, and this signal is inputted into the optical elementdriving circuit 57. Based on the inputted signal, the optical elementdriving circuit 57 outputs a signal to the optical element 100 that isnecessary to correct the wavefront aberration generated when the actualbase material thickness of the optical recording medium 60 deviates fromthe design value. The light that enters the optical element 100 isimparted with a wavefront aberration based on the signal outputted bythe optical element driving circuit 57. More specifically, depending onthe sign of the deviation of the base material thickness, a wavefrontaberration is imparted such that a phase (power component) that convertsparallel light into divergent light, or a phase that converts parallellight into convergent light, is imparted.

Next, light that has passed through the optical element 100 enters theλ/4 plate 93, and is converted from linearly polarized light intocircularly polarized light. Because this circularly polarized lightenters the objective lens 55 in a non-parallel state, sphericalaberration is caused by the objective lens 55, and this sphericalaberration corrects the spherical aberration generated by a deviation inthe base material thickness of the optical recording medium 60.Consequently, a light spot is formed on the optical recording medium 60that is without aberration, that is to say, has been constricted to thediffraction limit.

Light that enters the optical recording medium 60 is reflected by theoptical recording medium 60, and becomes light with a wavefrontaberration that is caused when there is a deviation in the base materialthickness of the optical recording medium 60. This light passes throughthe objective lens 55 and enters the λ/4 plate 93. Light that hasentered the λ/4 plate 93 is converted from circularly polarized lightinto linearly polarized light. The polarization direction of thelinearly polarized light is perpendicular with respect to the linearlypolarized light emitted from the light source 51. This linearlypolarized light, which has spherical aberration, enters the opticalelement 100 of the present invention, and its spherical aberration iscorrected by imparting a same power component as on the incoming lightpath.

The linearly polarized light that has passed through the optical element100 is diffracted approximately 100% by the polarized holographicoptical element 91, and the +1 order light of the diffraction enters thefirst photodetector 58, whereas the −1 order light of the diffractionenters the second photodetector 59. The first photodetector 58 outputs afocus error signal, which indicates the focusing condition of the lighton the optical recording medium 60, and a tracking error signal, whichindicates the irradiation position of the light. The secondphotodetector 59 outputs a signal regarding information recorded on theoptical recording medium 60.

The focus error signal outputted from the first photodetector 58 isgiven to a focus control circuit, which is not shown in the drawings.The focus control circuit controls the position of the objective lens 55in the direction of the optical axis, based on the focus error signal,such that light is focused on the optical recording medium 60 in a stateof focus. Furthermore, the tracking error signal is given to a trackingcontrol circuit, which is not shown in the drawings. The trackingcontrol circuit controls the position of the objective lens 55, based onthe tracking error signal, such that it focuses the light on the desiredtrack of the optical recording medium 60. The position of the objectivelens 55 is controlled using actuators 62.

Because an optical polarization system is used in the optical head 90,the usage efficiency of the light that is emitted by the light source 51is high, and recording/reproducing of rewritable optical recording mediabecomes easy.

The following is an explanation of the optical element 100 of thepresent invention. Liquid crystals are uniaxially birefringentmaterials, so that phase changes can be imparted on incident light onlywhen the rubbing direction of the liquid crystal is parallel to thepolarization direction of the light. If the polarization directions forthe incoming light path and the returning light path are perpendicular,as in an optical polarization system, then it is not possible to imparta phase change in the returning light path with the optical element ofthe Embodiment 1. In the optical head of the present invention, whichcarries out the correction with a combination of an optical element thatdefocuses the incident light and an objective lens, there is the problemthat if the same phase change is not imparted in the incoming light pathand the returning light path, then the light on the photodetectors isdefocused. Thus, in order to impart the same phase change on thepolarized returning light as well, an optical element that is adapted tooptical polarization systems has to be used for the optical head of theEmbodiment 3.

The following is a description of the optical element 100. FIG. 10 showsa schematic cross-sectional view of the optical element 100. The opticalelement 100 includes a first substrate 101, a second substrate 102, athird substrate 103, a first voltage application electrode 104, a secondvoltage application electrode 105, a first opposing electrode 106, asecond opposing electrode 107, a first translucent resin film 108, asecond translucent resin film 109, a third translucent resin film 110, afourth translucent resin film 111, a first liquid crystal 112, a secondliquid crystal 113, a first sealing resin 114, and a second sealingresin 115.

The first, second and third substrates 101, 102 and 103 are arrangedsubstantially parallel with respect to one another. These substrates aretranslucent, and made of glass, for example.

The first voltage application electrode 104 is disposed between thefirst substrate 101 and the first liquid crystal 112. The first voltageapplication electrode 104 is for applying a desired voltage to the firstliquid crystal 112. The first voltage application electrode 104 isformed on the principal surface on the inner side (liquid crystal 112side) of the first substrate 101.

The second voltage application electrode 105 is disposed between thethird substrate 103 and the second liquid crystal 113. The secondvoltage application electrode 105 is for applying a desired voltage tothe second liquid crystal 113. The second voltage application electrode105 is formed on the principal surface on the inner side (liquid crystal113 side) of the third substrate 103.

The first and the second voltage application electrodes 104 and 105include a plurality of segment electrodes, as shown in FIG. 2. It shouldbe noted that if the voltage application electrodes or the opposingelectrodes are formed on curved surfaces as shown in FIG. 3, they areformed continuously.

The first opposing electrode 106 is arranged substantially parallel tothe first voltage application electrode 104. Together with the firstvoltage application electrode 104, the first opposing electrode 106applies a desired voltage to the first liquid crystal 112. The firstopposing electrode 106 is formed substantially uniformly on at leastthat part of the principal surface of the second substrate 102 on theside of the second liquid crystal 112 that is in opposition to thesegment electrodes.

The second opposing electrode 107 is arranged substantially parallel tothe second voltage application electrode 105. Together with the secondvoltage application electrode 105, the second opposing electrode 107applies a desired voltage to the second liquid crystal 113. The secondopposing electrode 107 is formed substantially uniformly on at leastthat part of the principal surface of the second substrate 102 on theside of the second liquid crystal 113 that is in opposition to thesegment electrodes.

The first and the second translucent resin films 108 and 109 are formedcovering the first voltage application electrode 104 and the firstopposing electrode 106, respectively. The first and the secondtranslucent resin films 108 and 109 are alignment films for aligning thefirst liquid crystal 112 into a predetermined direction, and are made,for example, of polyvinyl alcohol films. By rubbing the first and thesecond translucent resin films 108 and 109, the first liquid crystal 112can be aligned in a predetermined direction.

The third and the fourth translucent resin films 110 and 111 are formedcovering the second voltage application electrode 105 and the secondopposing electrode 107, respectively. The third and the fourthtranslucent resin films 110 and 111 are alignment films for aligning thesecond liquid crystal 113, and are made, for example, of polyvinylalcohol films. By rubbing the third and the fourth translucent resinfilms 110 and 111, the second liquid crystal 113 can be aligned in apredetermined direction. The alignment direction of the first liquidcrystal 112 is perpendicular to the alignment direction of the secondliquid crystal 113.

The first liquid crystal 112 is disposed between the first and secondtranslucent resin films 108 and 109 (between the first voltageapplication electrode 104 and the first opposing electrode 106). Thefirst liquid crystal 112 functions as a phase changing layer thatchanges the phase of the incident light. For the first liquid crystal112, it is possible to use a nematic liquid crystal, for example. Bychanging the voltage between the first voltage application electrode 104and the first opposing electrode 106, the refractive index of the firstliquid crystal 112 can be changed, which makes it possible to change thephase of the incident light.

The second liquid crystal 113 is disposed between the third and fourthtranslucent resin films 110 and 111 (between the second voltageapplication electrode 105 and the second opposing electrode 107). Thesecond liquid crystal 113 functions as a phase changing layer thatchanges the phase of the incident light. For the second liquid crystal113, it is possible to use a nematic liquid crystal, for example. Bychanging the voltage between the second voltage application electrode105 and the second opposing electrode 107, the refractive index of thesecond liquid crystal 113 can be changed, which makes it possible tochange the phase of the incident light.

The first sealing resin 114 seals the first liquid crystal 112, and isarranged between the first and the second translucent resin films 108and 109, so that it encloses the first liquid crystal 112. The secondsealing resin 115 seals the second liquid crystal 113, and is arrangedbetween the third and the fourth translucent resin films 110 and 111, sothat it encloses the second liquid crystal 113. It is possible to use,for example, an epoxy resin for the first and the second sealing resins114 and 115.

The following explains how the optical element 100 is operated. Acontrol voltage is applied from the outside to the segment electrodes ofthe first and the second voltage application electrodes 104 and 105. Inthis situation, the alignment direction of the first liquid crystal 112is perpendicular to the alignment direction of the second liquid crystal113, so that the linearly polarized light of the incoming light path isaffected only by the refractive index change of the first liquid crystal112, and phases of the power components can be imparted with the firstliquid crystal 112. That is to say, the plane wave incident on theoptical element 100 is converted into a spherical wave.

On the other hand, the light that is reflected by the optical recordingmedium 60 is linearly polarized light that is perpendicular to theincoming linearly polarized light. Therefore, this returning light isaffected only by the second liquid crystal 113, and phases of the powercomponents can be imparted with the second liquid crystal 113. Now, ifthe pattern of the first and the second voltage application electrodes104 and 105 and the applied voltages are the same, then the same phasecan be imparted at the incoming path and the returning path, so that thespherical wave incident on the optical element 100 can be converted intoa plane wave.

In this manner, spherical aberration can be corrected also for opticalpolarization systems by using two liquid crystal layers with differentalignment directions. By using the optical element 100 to perform thecorrection, favorable lens shift characteristics can be attained, asexplained for Embodiment 2.

It should be noted that Embodiment 3 has been explained for the casethat the optical element is provided with two liquid crystal layers, butit is also possible to arrange optical elements as explained inEmbodiment 1 such that their alignment directions of the liquid crystalare perpendicular.

Embodiment 4

In Embodiment 4, an example of an optical recording/reproducingapparatus of the present invention is described. The opticalrecording/reproducing apparatus of Embodiment 4 records or reproducessignals on an optical recording medium, but it also can record andreproduce signals.

FIG. 11 schematically shows the configuration of an opticalrecording/reproducing apparatus 116 of Embodiment 4. Referring to FIG.11, the optical recording/reproducing apparatus 116 includes an opticalhead 50, an optical element driving circuit 57, a motor 117, and aprocessing circuit 118.

The optical head 50 is the one explained in Embodiment 2, and includesan optical element of the present invention. Instead of the optical head50, it is also possible to use the optical head 90. Redundantexplanations regarding these optical heads have been omitted.

In the optical recording/reproducing apparatus 116 of Embodiment 4, asemiconductor laser element emitting a laser beam with a wavelength inthe range of 390 nm to 420 nm is used as the light source. For theobjective lens, an objective lens with a NA in the range of 0.7 to 0.9is used.

The following explains how the optical recording/reproducing apparatus116 operates. First, when an optical recording medium 60 is set in theoptical recording/reproducing apparatus 116, the processing circuit 118gives out a signal that lets the motor 117 rotate, and the motor 117starts to rotate. Then, the processing circuit 118 drives the lightsource 51, causing it to emit light. The light emitted by the lightsource 51 is reflected by the optical recording medium 60, and entersthe first and the second photodetectors 58 and 59.

The first photodetector 58 outputs a focus error signal, which indicatesthe focusing condition of the light on the optical recording medium 60,and a tracking error signal, which indicates the irradiation position ofthe light, to the processing circuit 118. Based on these signals, theprocessing circuit 118 outputs a signal controlling the actuators 62,whereby light emitted by the light source 51 is focused on the desiredtrack of the optical recording medium 60. Based on the output from thesecond photodetector 59, the processing circuit 118 reproduces theinformation that has been recorded on the optical recording medium 60.

The following is an explanation of the control when the actual basematerial thickness of the optical recording medium 60 differs from thedesign value. In the optical recording/reproducing apparatus 116, theoptical head is configured such that correction of spherical aberrationis unnecessary if the base material thickness of the optical recordingmedium 60 is the same as its design value. Therefore, correction ofspherical aberration is necessary when the actual base materialthickness deviates from the design value.

If the actual base material thickness of the optical recording medium 60deviates from the design value, then the base material thickness sensor56 outputs a signal corresponding to the deviation of the base materialthickness of the optical recording medium 60 to the processing circuit118. The processing circuit 118 controls the optical element drivingcircuit 57 in accordance with this received signal, so that the controlsignal that is necessary to correct the spherical aberration caused bythe deviation of the base material thickness of the optical recordingmedium 60 is given by the optical element driving circuit 57 to theoptical element 54. Then, the spherical aberration is corrected by theoptical element 54 (see Embodiments 1 and 2 for details). Thus, theinformation signals recorded on the optical recording medium 60 can bereproduced correctly, even when the base material thickness of theoptical recording medium 60 deviates from the design value.

Embodiment 4 has been explained as an apparatus detecting the deviationof the base material thickness with a base material thickness sensor,but the optical recording/reproducing apparatus of the present inventionis not limited to an apparatus using a base material thickness sensor.For example, it is also possible to learn the deviation of the basematerial thickness when setting the optical recording medium 60 on themotor, and to correct the spherical aberration based on the learneddeviation of the base material thickness.

Embodiment 5

In Embodiment 5, another example of an optical recording/reproducingapparatus of the present invention and a method for opticalrecording/reproducing using the same are described. The opticalrecording/reproducing apparatus and method of Embodiment 5 can record orreproduce signals with a first optical recording medium having only onerecording layer or a second optical recording medium having a pluralityof recording layers.

The optical recording/reproducing apparatus of Embodiment 5 includes alight source and a spherical aberration correction means arrangedbetween the optical recording medium and the light source. Morespecifically, an optical recording/reproducing apparatus including anoptical head in accordance with the present invention can be used. Inthat case, the optical element of the present invention and theobjective lens function as the spherical aberration correction means. Inthe optical recording/reproducing apparatus of Embodiment 5, asemiconductor laser element emitting a laser beam with a wavelength inthe range of 390 nm to 420 nm is used as the light source. For theobjective lens, an objective lens with a NA in the range of 0.7 to 0.9is used.

The following is an explanation for the case that the opticalrecording/reproducing apparatus 116 described in Embodiment 4 is used asthe optical recording/reproducing apparatus of Embodiment 5.

FIG. 12A schematically illustrates the base material thickness of afirst optical recording medium 121 and a second optical recording medium122 for recording or reproducing with the optical recording/reproducingapparatus 116. The optical recording medium 121 includes only arecording layer A as the recording layer. The optical recording medium122 includes recording layers B and C as the recording layers. It shouldbe noted that the optical recording medium 122 can include two or morerecording layers.

The base material thickness for the recording layer A (that is, thedistance from the surface 121 s to the recording layer A) is denoted by“a” in FIG. 12A. The base material thicknesses for the recording layersB and C (that is, the distances from the surface 122 s to the recordinglayers B and C) are denoted respectively by “b” and “c” in FIG. 12B. Inthe optical recording/reproducing apparatus of Embodiment 5, one basematerial thickness of the second optical recording medium 122 (i.e. “b”or “c”) is equivalent to the base material thickness “a”. FIG. 12Aillustrates the case that the base material thickness “a” is equivalentto the base material thickness “b”. These base material thicknesses arethe total thickness of the substrate and the layers formed between thesubstrate and the recording layer (such as a UV cured resin). Therecording layer is made of a phase-changing material whose refractiveindex changes when a phase change between the crystalline phase and thenoncrystalline phase occurs.

It is preferable that the administrative information of the secondoptical recording medium 122 is recorded in the second recording layerB, for which the base material thickness is equivalent to the basematerial thickness “a”. With this configuration, the administrativeinformation of the optical recording medium 122 can be reproduced withthe spherical aberration correction means still in the initial state.

In the optical recording/reproducing apparatus of Embodiment 5, theoptical head 50 is designed to be adapted to the base material thickness“a” of the first optical recording medium 121. That is to say, theoptical head 50 is designed such that it has such a margin thatreproduction is possible without performing a correction of thespherical aberration for deviations of the base material thickness “a”of the first optical recording medium 121. In this case, the correctionof spherical aberration becomes necessary for deviations of the basematerial thickness of the second optical recording medium 122.

The following explains how the optical recording/reproducing apparatus116 of Embodiment 5 operates. First, when the optical recording medium121 or 122 is set on the motor 117, the processing circuit 118 lets themotor 117 rotate. Then, in the initial state before performing recordingor reproducing, the processing circuit 118 drives the sphericalaberration correction means such that it corrects the sphericalaberration for the base material thickness “a” without deciding whetherthe set optical recording medium is the optical recording medium 121 or122. More specifically, an external voltage is applied to the opticalelement of the present invention.

Then, focusing control is carried out with a focus control means. Thefocus control means is constituted by the actuators 62 and theprocessing circuit 118.

The following is an explanation of a method for focus control. First,light is emitted by driving the light source 51, and the light reflectedby the optical recording medium 60 is detected with the firstphotodetector 58. The first photodetector 58 outputs to the processingcircuit 118 a focus error signal, which indicates the focusing conditionof the light on the optical recording medium 60, and a tracking errorsignal, which indicates the irradiation position of the light. Based onthese signals, the processing circuit 118 outputs a signal controllingthe objective lens 55, whereby light emitted by the light source 51 isfocused on the desired track of the optical recording medium 60. Basedon the signal from the second photodetector 59, the processing circuit118 reproduces the information that has been recorded on the opticalrecording medium 60.

On the other hand, if recording or reproducing is performed with therecording layer C of the second optical recording medium 122, then thespherical aberration correction means is driven such that the sphericalaberration of the recording layer C is corrected. More specifically, atsubstantially the same time when the processing circuit 118 outputs thesignal shifting the focus from the recording layer B to the recordinglayer C, the processing circuit 118 drives the optical element drivingcircuit 57 such that it outputs a signal correcting the deviation of thebase material thickness. With this configuration, the signal is recordedor reproduced favorably, even when changing the recording layer used forrecording or reproducing.

Thus, with the optical recording/reproducing apparatus of Embodiment 5,signals are recorded or reproduced with an optical recording/reproducingmethod including a first step of driving the spherical aberrationcorrection means such that spherical aberration of the recording layer Ais corrected, before the recording or reproducing. If signals arerecorded or reproduced with the recording layer C, then this opticalrecording/reproducing method includes a second step of driving thespherical aberration correction means such that spherical aberration ofthe recording layer C is corrected, which is carried out after the firststep. That is to say, the optical recording/reproducing apparatus of theEmbodiment 5 stores in the processing circuit 118 a program for carryingout this optical recording/reproducing method.

In the optical recording/reproducing apparatus and method of Embodiment5, spherical aberration can be corrected without deciding whether theoptical recording medium has one or whether it has a plurality ofrecording layers, so that the time until recording or reproducing can beshortened. It should be noted that even if it is known whether theoptical recording medium has only one or a plurality of recordinglayers, the focus control begins with the spherical aberrationcorrection means adapted to the base material thickness “a”. Also inthis case, the time until the focusing control is shortened, because thespherical aberration correction means is in its initial state.

If the wavelength is shortened and the NA is increased to achieve higherdensities, then the margin with respect to spherical aberration becomessmaller. Therefore, if the base material thickness of the recordinglayer differs, it becomes necessary to correct spherical aberration foreach recording layer. In that case, assuming that the base materialthicknesses of the recording layers A, B and C are different from oneanother, correction of spherical aberration is not necessary for therecording layer A, but correction of spherical aberration is necessaryfor the recording layers B and C. On the other hand, making the basematerial thickness of the recording layer A equivalent to the basematerial thickness of the recording layer B as described above, it issufficient when spherical aberration is corrected only for the recordinglayer C. Therefore, in the optical recording/reproducing apparatus ofEmbodiment 5, the circuit for performing the correction of sphericalaberration can be simplified. Also, when the base material thickness ofthe recording layers is learned, the base material thickness of onelayer is known, so that the learning time can be shortened.

Moreover, with the optical recording/reproducing apparatus of Embodiment5, an optical recording/reproducing apparatus reproducing with highreliability information signals recorded on the optical recording mediumcan be attained by using the optical element of the present invention.Furthermore, the tolerance with respect to deviations of the basematerial thickness of the optical recording medium 60 can be increasedby using the optical element of the present invention, so that anoptical recording/reproducing apparatus is attained that can bemanufactured easily and at low cost.

Embodiment 6

In Embodiment 6, another example of an optical recording/reproducingapparatus of the present invention and an optical recording/reproducingmethod using the same are described. The optical recording/reproducingapparatus and method of Embodiment 6 can record or reproduce signalswith a first optical recording medium having only one recording layer orwith a second optical recording medium having a plurality of recordinglayers.

The optical recording/reproducing apparatus of Embodiment 6 includes alight source, a spherical aberration correction means arranged betweenthe optical recording medium and the light source, a focus errordetection means, and a focus control means. More specifically, anoptical recording/reproducing apparatus including an optical head inaccordance with the present invention can be used.

In the following explanations, the optical recording/reproducingapparatus 116 explained in Embodiment 4 is used for the opticalrecording/reproducing apparatus of the Embodiment 6. In this case, theoptical element 54 of the present invention and the objective lens 55function as the spherical aberration correction means, the firstphotodetector 58 functions as the focus error detection means, and theactuators 62 and the processing circuit 118 function as the focuscontrol means. In the optical recording/reproducing apparatus ofEmbodiment 6, a semiconductor laser element emitting a laser beam with awavelength in the range of 390 nm to 420 nm is used as the light source.For the objective lens, an objective lens with a NA in the range of 0.7to 0.9 is used.

In Embodiment 6, the base material thicknesses of the first opticalrecording medium and the second optical recording medium that arerecorded or reproduced with the optical recording/reproducing apparatus116 differ from one another. When “a” is the base material thickness ofthe first optical recording medium, and “b” and “c” are the basematerial thicknesses of the recording layers of the second opticalrecording medium, then “a”, “b” and “c” differ from one another (seeFIG. 12B).

With the optical recording/reproducing apparatus of Embodiment 6, in theinitial state before recording or reproducing signals on the opticalrecording medium, the spherical aberration correction means is driven soas to correct spherical aberration of the recording layer included bythe first optical recording medium. Then, the focus error is detectedwith the focus error detection means, and focus control is carried outwith the focus control means, based on the detected focus error.Recording and reproducing of the signals is performed after the focuscontrol. For the method for correcting spherical aberration, the methodfor detecting the focus error, and the method for focus control, it ispossible to use the methods explained in the above-describedembodiments.

Thus, with the optical recording/reproducing apparatus of Embodiment 6,signals are recorded or reproduced with an optical recording/reproducingmethod that includes a first step of driving the spherical aberrationcorrection means such that spherical aberration of the recording layerincluded in the first optical recording medium is corrected, beforerecording or reproducing. This optical recording/reproducing method alsoincludes a second step, carried out after the first step, of detectingthe focus error with the focus error detection means, and a third stepof performing focus control with the focus control means based on thedetected focus error signal. The recording or reproducing of signals iscarried out after these three steps. If the optical recording medium setin the apparatus is a second optical recording medium including aplurality of recording layers, then, after finishing these steps, thespherical aberration correction means is driven based on a base materialthickness error signal obtained with the base material thickness sensor,and the recording or reproducing is performed after that. In the opticalrecording/reproducing apparatus of Embodiment 6, the processing circuit118 stores a program for carrying out this optical recording/reproducingmethod.

In the optical recording/reproducing apparatus and the opticalrecording/reproducing method of Embodiment 6, the spherical aberrationcorrection means is driven and focus control is performed to be adaptedto the base material thickness of the recording layer of the firstoptical recording medium, regardless of whether the optical recordingmedium set in the optical recording/reproducing apparatus is a firstoptical recording medium or a second optical recording medium (that is,regardless of whether it is already known or not if the opticalrecording medium set in the optical recording/reproducing apparatus is afirst optical recording medium or a second optical recording medium).With this configuration, focus control can be carried out withoutdetermining whether the optical recording medium to be recorded orreproduced is an optical recording medium with only one recording layeror an optical recording medium with a plurality of recording layers, sothat the time until focus control can be shortened.

It should be noted that if it is known whether the optical recordingmedium subjected to recording or reproducing is a first opticalrecording medium or a second optical recording medium, then it is alsopossible to drive the spherical aberration correction means such thatthe spherical aberration at a standard base material thickness of therecording layer subjected to recording or reproducing is corrected.Then, the focus error can be detected with the focus error detectionmeans, and focus control can be performed with the focus control means,based on the detected focus error. Recording or reproducing areperformed after the focus control. With this method, the sphericalaberration correction means is driven so as to correct the sphericalaberration at a standard base material thickness before the focuscontrol. Therefore, the deviation of the actual base material thicknessfrom the standard base material thickness is detected with the basematerial thickness sensor, and recording or reproducing can be performedafter complete correction of the spherical aberration based on thatdeviation.

That is to say, in Embodiment 6, recording or reproducing is performedby an optical recording/reproducing method including a first step ofacquiring the information whether the optical recording medium subjectedto recording or reproducing is a first optical recording medium or asecond optical recording medium. This optical recording/reproducingmethod also includes a second step of driving the spherical aberrationcorrection means so as to correct spherical aberration at the standardbase material thickness of the recording layer subjected to recording orreproducing, based on the acquired information, a third step ofdetecting the focus error with the focus error detection means, and afourth step of performing focus control with the focus control meansbased on the detected focus error signal. The recording or reproducingthen is performed after the focus control. In this case, the processingcircuit 118 stores a program including these four steps.

In the first step, the information whether the optical recording mediumsubjected to recording or reproducing is a first optical recordingmedium with one recording layer or a second optical recording mediumwith a plurality of recording layers is acquired as follows. Focus errorsignals indicating the focus error are detected in correspondence withthe recording layers. This means, if there are two recording layers,then two focus error signals are detected. Thus, if a circuit isconfigured that detects the number of detected signals, then it ispossible to acquire the information of how many recording layers theoptical recording medium has.

With this method of initially acquiring the information whether theoptical recording medium subjected to recording or reproducing is anoptical recording medium having only one recording layer or an opticalrecording medium having a plurality of recording layers, the focuscontrol is performed after spherical aberration correction in accordancewith the standard base material thickness depending on the recordinglayer, so that a smoother focus control becomes possible.

This embodiment has been explained for the case of using an opticalrecording/reproducing apparatus including the optical head 50, but it isalso possible to use the optical head 90.

Furthermore, the above embodiments have been explained for the case thatthe optical element of the present invention is arranged between thecollimating lens and the objective lens (parallel optical system), butthe optical element also can be arranged between the light source andthe collimating lens (divergent optical system).

Furthermore, the above embodiments have been explained for an opticalhead of an infinite system, but it is also possible to use an opticalhead of an finite system without a collimating lens for the optical headof the present invention.

Furthermore, the Embodiments 2 and 3 have been explained for the casethat the deviation of the base material thickness is measured with asensor, but it is also possible to correct the spherical aberrationusing a deviation of the base material thickness that has been learned.

Furthermore, the above embodiments have been explained for the case thata diffraction grating is used to separate the light reflected by theoptical recording medium from the light path from the light source, butinstead of the diffraction grating, it is also possible to use anotheroptical element (for example, a half mirror).

Furthermore, the above embodiments have been explained for an opticalrecording medium recording information only optically, but the presentinvention also can be applied to optical recording media, in whichinformation is recorded optically and magnetically. Furthermore, thepresent invention is not limited to disk-shaped optical recording media,and also can be applied to card-shaped optical recording media, forexample.

Furthermore, the above embodiments have been explained for opticalelements changing the phase of transmitted light, but it is alsopossible to use an optical element that changes the phase when incidentlight is reflected. For example, it is possible to use an opticalelement with a phase changing layer made of piezoelectric material, andto change the phase of the reflected light by distorting the phasechanging layer.

Furthermore, the above-described optical recording/reproducingapparatuses and optical recording/reproducing methods have beenexplained for the case of using an optical element of the presentinvention and an objective lens as the spherical aberration correctionmeans, but it is also possible to use other spherical aberrationcorrection means. For example, two lenses arranged along the opticalaxis can be used as the spherical aberration correction means. Sphericalaberration then can be corrected by moving these two lenses on theoptical axis.

Furthermore, the above-described optical recording/reproducingapparatuses and optical recording/reproducing methods have beenexplained for the case of using a photodetector as the focus errordetection means, but it is also possible to use other focus errordetection means.

Furthermore, the above-described optical recording/reproducingapparatuses and optical recording/reproducing methods have beenexplained for the case of using a processing circuit and actuators asthe focus control means, but it is also possible to use other focuscontrol means.

Furthermore, the above-described embodiments have been explained for thecase of using a semiconductor laser as the light source, but it is alsopossible to use, for example, an SHG light source generating secondharmonics as the light source.

Furthermore, the above-described embodiments have been explained for thecase of using a phase changing optical recording medium as the opticalrecording medium, but it is also possible to use other optical recordingmedia, such as optomagnetic recording media.

As explained above, with the optical element of the present invention,an optical head can be configured in which the correctional effect doesnot deteriorate when the objective lens is shifted. Therefore, with anoptical head using the optical element of the present invention, a firstoptical recording/reproduction apparatus, or a first opticalrecording/reproduction method, signals can be recorded or reproducedwith high reliability.

Furthermore, with a second, third, or fourth opticalrecording/reproduction apparatus of the present invention, or a secondor third optical recording/reproduction method of the present invention,the time until recording or reproducing can be shorted, and theapparatus can be simplified.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription. All changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

1-26. (canceled)
 27. An optical element, comprising: a first voltageapplication electrode; a first opposing electrode arranged in oppositionto the first voltage application electrode; and a first phase changinglayer arranged between the first voltage application electrode and thefirst opposing electrode; wherein by changing a voltage between thefirst voltage application electrode and the first opposing electrode, aphase that converts plane waves into spherical waves is imparted onlight that is incident on the first phase changing layer.
 28. Theoptical element according to claim 27, wherein at least one electrodeselected from the first voltage application electrode and the firstopposing electrode is arranged on a curved surface.
 29. The opticalelement according to claim 27, wherein the first phase changing layer ismade a material whose refractive index changes when applying a voltage.30. The optical element according to claim 27, wherein the first phasechanging layer is made of a liquid crystal.
 31. The optical elementaccording to claim 27, wherein the first phase changing layer is made ofa material whose volume changes when subjected to voltage.
 32. Theoptical element according to claim 31, wherein the first phase changinglayer is made of PLZT.
 33. The optical element according to claim 27,wherein the first voltage application electrode includes a plurality ofsegment electrodes.
 34. The optical element according to claim 27,further comprising: a second voltage application electrode; a secondopposing electrode arranged in opposition to the second voltageapplication electrode; and a second phase changing layer arrangedbetween the second voltage application electrode and the second opposingelectrode; wherein, by changing a voltage between the second voltageapplication electrode and the second opposing electrode, a phase thatconverts plane waves into spherical waves is imparted on polarized lightthat is perpendicular to the polarization of the light that is incidenton the first phase changing layer.
 35. The optical element according toclaim 34, wherein at least one electrode selected from the secondvoltage application electrode and the second opposing electrode isarranged on a curved surface.
 36. The optical element according to claim34, wherein the second phase changing layer is made of a material whoserefractive index changes when applying a voltage between the secondvoltage application electrode and the second opposing electrode.
 37. Theoptical element according to claim 34, wherein the second phase changinglayer is made of a material whose volume changes when applying a voltagebetween the second voltage application electrode and the second opposingelectrode.
 38. The optical element according to claim 34, wherein thesecond voltage application electrode includes a plurality of segmentelectrodes.
 39. An optical head for recording or reproducing signals onan optical recording medium, the optical head comprising: a lightsource; an optical element arranged between the optical recording mediumand the light source; and an objective lens arranged between the opticalrecording medium and the optical element; wherein the optical element isthe optical element according to claim
 27. 40. The optical headaccording to claim 39, further comprising an N/4 wavelength plate(wherein N is an odd number of one or greater) arranged between theoptical element and the objective lens.
 41. An opticalrecording/reproducing apparatus for recording or reproducing signals onan optical recording medium, the optical recording/reproducing apparatuscomprising: an optical head for recording or reproducing signals on anoptical recording medium, the optical head comprising: a light source;an optical element arranged between the optical recording medium and thelight source; and an objective lens arranged between the opticalrecording medium and the optical element; wherein the optical element isthe optical element according to claim 27.